a. Field of the Invention
The present invention pertains generally to pyroelectric optical radiation detection, and more particularly to pyroelectric detectors constructed from a single electret in which the spontaneous polarization is selectively reversed to optimize acoustic nulling and/or modify spatial responsivity to enable position sensing.
b. Description of the Background
In the past, attempts have been made to reduce the unwanted microphonic signal in pyroelectric detectors by mechanical design, electrical design, or both. One such mechanical design places the electret on a platform optimized for dampening microphonic resonances. A platform, however, is not optimum because the low-frequency response of an electret of nonuniform thickness and/or heat sink conditions will not be spatially uniform. The simplest electrical design uses two identical electrets having opposite polarizations that are connected so that the effects of acoustic noise are cancelled. Multiple electrode schemes have been used on a single electret to reject microphonic signals. Domain engineering techniques have also been used. Domain engineering techniques in materials such as LiTaO3 have required the use of an ion mill. This has resulted in very small, fragile detectors. Domain engineering in polymers such as polyvinyladine flouride has also been used, but such detectors do not have the same pyroelectric coefficient as those made from LiNbO3 and LiTaO3. They also result in detectors that do not have a highly uniform thickness. Acoustic ringing can also result from the deformation of the electret as a result of sound or pressure waves, the impingement of a pulsed laser, and other sources of noise. High frequency pulsed lasers may also create acoustic waves that cause the thickness of the electret to vary, creating additional acoustic resonances. The acoustic ringing and resonances may take the form of standing or traveling waves in the electret.
It would therefore be advantageous to provide a single pyroelectric detector that provides acoustic nulling, beam detection and dampening of acoustic ringing and acoustic resonances.
The present invention overcomes the disadvantages and limitations of the prior art by providing a single pyroelectric detector that is capable of providing acoustic nulling, beam location detection and dampening of acoustic ringing and acoustic resonances. This is accomplished by using a single crystal that has reversed domain polarization regions. Acoustic nulling is provided by combining the outputs of the reversed domain polarization regions. Optical beam detection and location of the optical beam on the electret can then be realized as a result of the pyroelectric effects of the electret as opposed to the piezoelectric effects of acoustic noise. The reversed domain regions can be placed in a periodic pattern that is related to the speed at which the acoustic noise travels through the electret to allow nulling of standing or traveling waves. Additionally, acoustic noise is physically dissipated and scattered by the introduction of multiple domain regions that further reduces acoustic ringing. For example, the introduction of needle domains functions to scatter and dissipate acoustic waves in the electret.
The present invention may therefore comprise a method of reducing acoustic noise in a pyroelectric detector formed from a z-cut single crystal electret comprising: generating a plurality of domain regions in the electret having opposite polarization directions, the domain regions having a periodic pattern that substantially corresponds to wave patterns of acoustic noise; providing an electrode that covers approximately equal portions of the plurality of domain regions, so that charges generated by the plurality of domain regions in response to acoustic noise can be combined to substantially null the acoustic noise.
The present invention may further comprise the method of constructing a pyroelectric detector from a z-cut single crystal electret comprising: generating a first domain region in the electret having a first polarization; generating a plurality of needle domain regions having a polarization which is opposite to the first predetermined polarization.
The present invention may further comprise the method of constructing a multicell pyroelectric detector formed from a z-cut single crystal electret for detecting the position of a light beam comprising: generating at least two domain regions in a central portion of the electret having opposite polarizations; generating a plurality of needle domain regions surrounding the central portion of the electret.
The present invention may further comprise the method of forming a plurality of bicell pyroelectric detectors using a shadow mask comprising: providing a z-cut single crystal electret having a spontaneous polarization in a first direction; placing a shadow mask on the electret that has openings that correspond to the regions in which a polarization is desired that is opposite to the spontaneous polarization of the electret; depositing a poling electrode on the electret in the areas of the openings of the shadow mask; using the poling electrodes to polarize the electret with the opposite polarization; removing the poling electrodes; dividing up the electret to form the plurality of multicell pyroelectric detectors.
The present invention may further comprise a multicell pyroelectric detector formed by the method of: providing a z-cut single crystal electret having a spontaneous polarization in a first direction; placing a shadow mask on the electret that has openings that correspond to the regions in which a polarization is desired that is opposite to the spontaneous polarization of the electret; depositing a poling electrode on the electret in the areas of the openings of the shadow mask; using the poling electrodes to polarize the electret with the opposite polarization; removing the poling electrodes; dividing up the electret to form the plurality of multicell pyroelectric detectors.
The present invention may further comprise a pyroelectric detector formed from a z-cut single crystal electret comprising: a central portion of the electret; a plurality of domain regions surrounding the central portion the plurality of domain regions having opposite polarization directions, the plurality of domain regions having a periodic pattern that substantially corresponds to wave patterns of acoustic noise generated in the electret; an electrode that covers approximately equal portions of the plurality of domain regions so that charges generated by the plurality of domain regions in response to acoustic noise are combined to substantially null the acoustic noise.
The present invention may further comprise a pyroelectric detector that reduces acoustic noise comprising: a z-cut single crystal electret having a polarization in a first direction; at least one detector region disposed on the electret; a plurality of needle domain regions having a polarization that is opposite to the first direction, the needle domain regions dispersed on the electret around at least one detector region of the electret.
The advantages of the present invention are that acoustic noise can be nulled by combining the outputs of the oppositely poled regions of the electret in a single crystal. The use of the single crystal provides for better nulling for a number of reasons. For example, temperature variations between separate crystals do not occur using a single crystal. Additionally, the domain region interfaces function to dissipate and scatter the wavefronts of acoustic noise and reduces the acoustic ringing and resonances that may occur in the crystal. The present invention also uses a plurality of alternating oppositely polarized domain regions that are disposed in a geometrical pattern having a size that allows for nulling of at least the first order structural resonances in the crystal. The oppositely polarized domain regions are sized to be an integer value, e.g. a fractional integer value or a multiple integer value of the wavelength of the propagation speed of the acoustic signal through the electret, so that cancellation can be employed using the oppositely polarized regions.
Further, the present invention can be utilized to reduce acoustic noise that is generated by radiation pressure from pulsed lasers that are detected by the electret. Pulsed lasers can create a periodic ringing in the electret that is related to the frequency of the pulses of the pulsed laser, the propagation speed of acoustic waves in the electret, the size and aspect ratios of the electret and other factors. Interference patterns in the form of standing and traveling acoustic waves created by the pulsed laser are physically dispersed by the domain interfaces from both geometrically placed domain regions and needle domains. In addition to the physical scattering of the acoustic wave by the domain interfaces, the domain regions can be placed on the electret at some integer fraction or multiple of the wavelength of the standing or traveling acoustic wave produced by the pulsed laser, in the same fashion as indicated above, to provide for cancellation of these standing waves. In this fashion, if the pulsed frequency of the laser is known, as well as the speed of propagation of the acoustic signal within the crystalline electret material, domain regions can be designed to reduce the effects of these types of acoustic resonances.